sketch A - Engine well above waterline

sketch B - Engine just above waterline - fit high riser

sketch C - Engine just above waterline

sketch D - Engine near or below waterline

TYPICAL SYSTEM CONFIGURATIONS

Where's the Waterline?

The postion of the engine relative to the waterline is crucial to determining the most suitable exhaust configuration.

The engine’s cooling water is usually added to the exhaust right behind the engine so the following must be considered when designing a system:

• Will water run naturally out of the exhaust?
• Will it drain when you stop the engine?
• Might it collect in the exhaust and run back into the engine if the boat pitches?
• Is the exhaust system diameter large enough to allow free flow without creating high back pressure?

Engine well above waterline? (sketch A)

This engine is sited well above the waterline, with a gradient steeper than 1 in 8 to the back of the boat. The water will run naturally down the exhaust to the stern. To reduce noise by up to 50% a straightforward in-line silencer should be used. To reduce noise by around 70% a dual chamber unit could also be used.

Engine only just above waterline? (sketches B & C)

A problem! The hose from the engine to the transom is nearly flat. Water could run back into the engine if the vessel pitches on its mooring – or a wave could do the same. A "high riser" is needed on the engine. This is an extension to the exhaust outlet on the engine taking it well above water-line to create a 1 in 8 gradient from the water injection on the engine down to the stern. An In-Line silencer can then be used with a noise reduction of around 50%. Alternatively you might get the necessary height by using a Dual Chamber silencer, with a noise reduction of around 75%. We make high risers to order and some engine manufacturers also keep them as an option.

Engine below waterline? (sketch D)

Sailing yachts and many displacement craft have engines where the exhaust water injection point is actually below waterline. In this situation a lift silencer has to be put in the system to collect water when the engine is stopped. A syphon-breaker should be fitted into the cooling water feed. The exhaust must rise to a U bend (or "gooseneck") well above the waterline, and then run down to the transom. In this way the lift silencer collects water when the engine is stopped, and the gooseneck prevents a stern wave flooding the system. A standard Lift Silencer will reduce noise by up to 50%. Better still, replace the gooseneck with a water separator and reduce noise by up to 80% - this will have the added advantage of reducing back pressure.

Typical Configurations -  Listing